1. Field
Apparatuses and methods consistent with the present exemplary embodiments relate to a light emitting driving apparatus and a method of controlling the same, and more particularly, to a light emitting driving apparatus and a method of controlling the same, for minimizing a load fluctuation range of a light emitting device.
2. Description of the Related Art
A conventional liquid crystal display (LCD) device is thick and lightweight and has low driving voltage and power consumption compared with other display devices and thus has been widely used. However, since the LCD device is a non-emitting device that is not capable of emitting light by itself, the LCD device requires a separate backlight for supplying light to a liquid display panel.
As a backlight light source of the LCD device, a cold cathode fluorescent lamp (CCFL), a light emitting diode (LED), etc. have mainly been used. A CCFL is disadvantageous in that the CCFL uses mercury (Hg), which can cause environmental pollution, has a low response speed and a low color gamut, and is inappropriate for a short, small, and light weight LCD panel.
On the other hand, an LED is advantageous as a backlight source because the LED does not use environmentally hazardous chemicals and is thus environment-friendly, and has impulse driving. In addition, the LED is advantageous because the LED has excellent color gamut, luminance, color temperature, etc. Furthermore, the color of the lights of the LED can randomly change by adjusting an amount of red, green, and blue LEDs, and is appropriate for a short, small, and light weight LCD panel. Accordingly, the LED has been widely used as a backlight source of an LCD panel, etc.
An LCD having a backlight which employs an LED requires a driving circuit for supplying a predetermined current to the LED. A pulse width modulation (PWM) type direct current (DC)-DC converter is mainly used in a driving circuit for driving the LED at predetermined current or voltage. FIG. 1 illustrates a circuit diagram of a conventional light emitting driving apparatus 10. FIGS. 2(a)-2(c) illustrate various examples of graphs of a current waveform characteristic according to a frequency of a conventional light emitting driving apparatus.
Referring to FIG. 1, a conventional light emitting driving apparatus 10 includes a light emitting device and a power source for supplying DC power to the light emitting device. In addition, a diode, a capacitor, and an inductor are disposed between the light emitting device and the power source, and a switch M is connected between one side of the inductor and one side of the power source. As shown in FIG. 1, the switch M is connected to a PWM adjuster integrated circuit (IC) for adjusting an on-off duty ratio of the switch M, according to a voltage detected from a voltage detecting resistor Rcs. In this case, an average value of inductor current has the same value as output current that flows in the light emitting device, according to the characteristics of the shape of a circuit.
Referring to FIGS. 2(a)-(c), there is illustrated a waveform of an inductor current according to the circuit illustrated in FIG. 1, when the circuit operates in a steady-state. Referring to FIG. 2(a), when the switch M is turned on, the inductor current increases, and when the switch M is turned off, the inductor current decreases. In this case, an average value of the inductor current is equal to an output current of the light emitting device. In addition, in order to control an average value of the output current using only information about the switch current of a turn-on period detected by the voltage detecting resistor Rcs, the PWM adjuster IC controls the turn-on period such that output current detected at a turn-on point in time and an average value of the output current detected at a turn-on point in time are a target current value.
However, according to a conventional light emitting device, when an output voltage or an input voltage varies, a switching frequency varies. That is, when a high voltage is applied to a light emitting device, a turn-on period of a switch is increased to lower a switching frequency. On the other hand, when a low voltage is applied to the light emitting device, the turn-on period of the switch is decreased to raise a switching frequency. FIG. 2(b) illustrates a case in which a ripple of an inductor current increases as a switching frequency decreases. FIG. 2(c) illustrates a case in which a ripple of an inductor current decreases as a switching frequency increases.
Accordingly, when a switching frequency decreases, a switching ripple of an output current remarkably increases, and when the switching frequency increases, a switching loss is generated and heat is generated from a circuit for a light emitting device.